Box 5 Medicare Wages is higher than Box 1 Wages. Is this an error on the W2?

[ChristinaS]

Box 5 is greater than Box 1 on many W2s. The most common reason relates to 401(k) contributions (W2, Box 12, Code D). They are subject to payroll tax, but not to federal income tax.

On many W2s, 401(k)s will explain this difference.